Search Results

You are looking at 1 - 10 of 11 items for

  • Author or Editor: William M. Lapenta x
  • All content x
Clear All Modify Search
M. Carrier, X. Zou, and William M. Lapenta

Abstract

An effort is made to increase the number of Advanced Infrared Sounder (AIRS) cloud-uncontaminated infrared data for regional mesoscale data assimilation and short-term quantitative precipitation forecast (QPF) applications. The cloud-top pressure from Moderate Resolution Imaging Spectroradiometer (MODIS) is utilized in combination with weighting functions (WFs) to develop a channel-based cloudy-data-removal algorithm. This algorithm identifies “clear channels” for which the brightness temperature (BT) values are not cloud contaminated. A channel-dependent cutoff pressure (COP) level is first determined based on the structure of the WF of each channel. It is usually below the maximum WF level. If the cloud top (as identified by a MODIS cloud mask) is above (below) the COP level of a channel, this channel is then deemed cloudy (clear) and removed (retained). Using this algorithm, a sizable increase of cloud-uncontaminated AIRS data can be obtained. There are more usable domain points for those channels with higher COP levels. A case study is conducted. It is shown that instead of having less than 20% AIRS clear-sky observations, the algorithm finds 80% (58%) of the AIRS pixels on which there are channels whose COP levels are at or above 300 hPa (500 hPa) and the BT data in these channels at these pixels are cloud uncontaminated. Such a significant increase of the usable AIRS cloud-uncontaminated data points is especially useful for regional mesoscale data assimilation and short-term QPF applications.

Full access
William M. Lapenta and Nelson L. Seaman

Abstract

On 27–28 February 1982 cyclogenesis occurred along a Carolina coastal front. As the relatively weak low pressure center developed and moved northeastward along the front, up to 30 cm of snow fell in 12 hours in the mountains of western Virginia and moderate icing persisted throughout 27 February in the Carolinas. On 28 February the mesoscale cyclone intensified more rapidly, turned east-northeast after passing Cape Hatteras, and gradually became a typical synoptic scale oceanic storm.

A nested version of the Penn State/NCAR mesoscale model with 35-km fine-mesh resolution is used to simulate the prestorm environment and subsequent cyclogenesis during a 36-h period. Evaluation of the numerical results indicates that the model successfully reproduced most principal synoptic and mesoscale features associated with this complex east coast cyclogenesis case, including storm path and intensification, coastal front structure, cold-air damming, circulations induced by a polar jet streak, low-level jets, and precipitation. In particular this study 1) provides an in-depth numerical examination of a case of east coast cyclogenesis in which entrance region jet streak dynamics provides the dominant upper-level support white only a weak baroclinic wave was approaching from the west, 2) reveals the existence of two moist airstreams fed by onshore flow from the marine boundary layer east of the coastal front (a southeasterly low-level jet and a rapidly rising “feeder” supporting the ascending branch of the polar jet streak's entrance circulation), 3) explores the origin, history, and significance of these moist airstreams to cyclogenesis (the southeasterly low-level jet supports inland precipitation while the rapidly ascending airstream contributes to heavy precipitation and failling pressure along the coast), and 4) demonstrates that both airstreams are well developed very early during the cyclogenesis before the midlevel baroclinic wave reaches the coast.

Full access
William M. Lapenta and Nelson L. Seaman

Abstract

On 27–28 February 1982 cyclogenesis occurred along a Carolina coastal front. Despite the relatively weak low pressure center typical of many coastal storms, this case produced widespread hazardous conditions—within 12 h up to 30 cm of snow fell in the mountains of western Virginia and moderate icing persisted throughout 27 February in the Carolinas. The event contained many mesoscale and synoptic-scale phenomena such as cold-air damming, coastal frontogenesis, upper- and lower-tropospheric jet streaks, a thermally direct vertical-transverse ageostrophic circulation, and heavy mixed precipitation.

A nested version of the PSU–NCAR three-dimensional mesoscale model with 35-km resolution successfully reproduced most principal synoptic and mesoscale feature associated with the event. This study presents a series of numerical experiments designed to examine the role of several physical processes on the evolution of and interaction between atmospheric phenomena having dithering scales, each of which contributed to the development of the storm. In particular, the physical processes studied include: 1) the role of diabatic heating associated with convective and grid-scale precipitation, 2) the role of a thermally direct transverse circulation about the entrance region of a strong polar jet streak and 3) modification of the marine planetary boundary layer by fluxes of heat and moisture over the Gulf Stream.

Of the three mechanisms investigated, the diabatic heating associated with precipitation is found to have the most significant impact on storm development. Without latent heating, cyclogenesis does not occur along the Carolina coastal front despite the presence of strong low-level baroclinicity and cyclonic vorticity. A less dramatic but still important relationship is found between storm formation and the other two physical mechanisms. The experiments indicate that the timing of storm development is delayed and the intensity weakened by reducing the strength of both the polar jet streak and fluxes over the Gulf Stream. In particular, weakening these processes disrupts the positive phase relationship between upper- and lower-tropospheric forcing in the last 12 h of the study. The three basic mechanisms are shown to affect the cyclogenesis by altering many of the important mesoscale features and processes that contribute to storm development, including the intensity of the vertical-transverse circulation around the jet streak, the location of the upward branch of the circulation, precipitation intensity, buoyancy of parcels advected over the coastal front, low-level and upper-level height falls associated with latent heating, and the southeasterly low-level jet over the coastal front.

Full access
Deborah K. Nykanen, Efi Foufoula-Georgiou, and William M. Lapenta

Abstract

A coupled modeling framework is used in this study to investigate the effect of subgrid-scale rainfall variability on the spatial structure of the evolving storm and on other surface variables and water and energy fluxes. The Fifth-Generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model coupled with the Biosphere–Atmosphere Transfer Scheme is combined with a dynamical/statistical scheme for statistically downscaling rainfall. Model simulations with and without including subgrid-scale rainfall variability are compared at the grid scale to quantify the propagation of small-scale rainfall heterogeneities through the nonlinear land–atmosphere system. It was found that including subgrid-scale rainfall variability (here on the order of 3 km) affects the spatial organization of the storm system itself, surface temperature, soil moisture, and sensible and latent heat fluxes. These effects were found to occur at spatial scales much larger than the scale at which rainfall variability was prescribed, illustrating the pronounced nonlinear spatial dynamics of the land–atmosphere system and its important role on hydrometeorological predictions.

Full access
Roy W. Spencer, William M. Lapenta, and Franklin R. Robertson

Abstract

Spatial fields of satellite-measured deep-layer temperatures are examined in the context of quasigeostrophic theory. It is found that midtropospheric geostrophic vorticity and quasigeostrophic vertical motions can be diagnosed from microwave temperature measurements of only two deep layers. The lower- (1000–400 hPa) and upper- (400–50 hPa) layer temperatures are estimated from limb-corrected TIROS-N Microwave Sounding Units (MSU) channel 2 and 3 data, spatial fields of which can be used to estimate the midtropospheric thermal wind and geostrophic vorticity fields. Together with Trenberth's simplification of the quasigeostrophic omega equation, these two quantities can be then used to estimate the geostrophic vorticity advection by the thermal wind, which is related to the quasigeostrophic vertical velocity in the midtroposphere.

Critical to the technique is the observation that geostrophic vorticity fields calculated from the channel 3 temperature features are very similar to those calculated from traditional, “bottom-up” integrated height fields from radiosonde data. This suggests a lack of cyclone-scale height features near the top of the channel 3 weighting function, making the channel 3 cyclone-scale “thickness” features approximately the same as height features near the bottom of the weighting function. Thus, the MSU data provide observational validation of the LID (level of insignificant dynamics) assumption of Hirshberg and Fritsch.

Full access
Matthew J. Carrier, Xiaolei Zou, and William M. Lapenta

Abstract

An adjoint sensitivity analysis is conducted using the adjoint of the hyperspectral radiative transfer model (RTM) that simulates the radiance spectrum from the Advanced Infrared Sounder (AIRS). It is shown, both theoretically and numerically, that the height of the maximum sensitivity of radiance in a channel could be higher or lower than the height of the maximum weighting function of that channel. It is shown that the discrepancy between the two heights is determined by the vertical structures of the atmospheric thermodynamic state. The sensitivity finds the level at which changes in temperature and/or moisture will have the largest influence on the simulated brightness temperature (BT), and the maximum weighting function (WF) height indicates the level where the model atmosphere contributes most significantly to the emission at the top of the atmosphere. Based on the above findings, an adjoint method for forecast verification using AIRS radiances is presented. In this method, model forecasts are first mapped into radiance space by an RTM so that they can be compared directly with the observed radiance values. The adjoint sensitivity analysis results are then used to connect the deviations of the model forecasts from observed radiances to the changes of temperature and moisture variables in model space. This adjoint sensitivity based model verification provides useful information on forecast model performances based on indirect observations from satellites.

Full access
Katherine M. LaCasse, Michael E. Splitt, Steven M. Lazarus, and William M. Lapenta

Abstract

High- and low-resolution sea surface temperature (SST) analysis products are used to initialize the Weather Research and Forecasting (WRF) Model for May 2004 for short-term forecasts over Florida and surrounding waters. Initial and boundary conditions for the simulations were provided by a combination of observations, large-scale model output, and analysis products. The impact of using a 1-km Moderate Resolution Imaging Spectroradiometer (MODIS) SST composite on subsequent evolution of the marine atmospheric boundary layer (MABL) is assessed through simulation comparisons and limited validation. Model results are presented for individual simulations, as well as for aggregates of easterly- and westerly-dominated low-level flows. The simulation comparisons show that the use of MODIS SST composites results in enhanced convergence zones, earlier and more intense horizontal convective rolls, and an increase in precipitation as well as a change in precipitation location. Validation of 10-m winds with buoys shows a slight improvement in wind speed. The most significant results of this study are that 1) vertical wind stress divergence and pressure gradient accelerations across the Florida Current region vary in importance as a function of flow direction and stability and 2) the warmer Florida Current in the MODIS product transports heat vertically and downwind of this heat source, modifying the thermal structure and the MABL wind field primarily through pressure gradient adjustments.

Full access
Jonathan L. Case, William L. Crosson, Sujay V. Kumar, William M. Lapenta, and Christa D. Peters-Lidard

Abstract

This manuscript presents an assessment of daily regional simulations of the Weather Research and Forecasting (WRF) numerical weather prediction (NWP) model initialized with high-resolution land surface data from the NASA Land Information System (LIS) software versus a control WRF configuration that uses land surface data from the National Centers for Environmental Prediction (NCEP) Eta Model. The goal of this study is to investigate the potential benefits of using the LIS software to improve land surface initialization for regional NWP. Fifty-eight individual nested simulations were integrated for 24 h for both the control and experimental (LISWRF) configurations during May 2004 over Florida and the surrounding areas: 29 initialized at 0000 UTC and 29 initialized at 1200 UTC. The land surface initial conditions for the LISWRF runs came from an offline integration of the Noah land surface model (LSM) within LIS for two years prior to the beginning of the month-long study on an identical grid domain to the subsequent WRF simulations. Atmospheric variables used to force the offline Noah LSM integration were provided by the North American Land Data Assimilation System and Global Data Assimilation System gridded analyses.

The LISWRF soil states were generally cooler and drier than the NCEP Eta Model soil states during May 2004. Comparisons between the control and LISWRF runs for one event suggested that the LIS land surface initial conditions led to an improvement in the timing and evolution of a sea-breeze circulation over portions of northwestern Florida. Surface verification statistics for the entire month indicated that the LISWRF runs produced a more enhanced and accurate diurnal range in 2-m temperatures compared to the control as a result of the overall drier initial soil states, which resulted from a reduction in the nocturnal warm bias in conjunction with a reduction in the daytime cold bias. Daytime LISWRF 2-m dewpoints were correspondingly drier than the control dewpoints, again a manifestation of the drier initial soil states in LISWRF. The positive results of the LISWRF experiments help to illustrate the importance of initializing regional NWP models with high-quality land surface data generated at the same grid resolution.

Full access
Richard T. McNider, William M. Lapenta, Arastoo P. Biazar, Gary J. Jedlovec, Ronnie J. Suggs, and Jonathan Pleim

Abstract

In weather forecast and general circulation models the behavior of the atmospheric boundary layer, especially the nocturnal boundary layer, can be critically dependent on the magnitude of the effective model grid-scale bulk heat capacity. Yet, this model parameter is uncertain both in its value and in its conceptual meaning for a model grid in heterogeneous conditions. Current methods for estimating the grid-scale heat capacity involve the areal/volume weighting of heat capacity (resistance) of various, often ill-defined, components. This can lead to errors in model performance in certain parameter spaces. Here, a technique is proposed and tested for recovering bulk heat capacity using time tendencies in satellite-retrieved surface skin temperature (SST). The technique builds upon sensitivity studies that show that surface temperature is most sensitive to thermal inertia in the early evening hours. The retrievals are made within the context of a surface energy budget in a regional-scale model [the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5)]. The retrieved heat capacities are used in the forecast model, and it is shown that the model predictions of temperature are improved in the nighttime during the forecast periods.

Full access
Thomas M. Hamill, Gary T. Bates, Jeffrey S. Whitaker, Donald R. Murray, Michael Fiorino, Thomas J. Galarneau Jr., Yuejian Zhu, and William Lapenta

A multidecadal ensemble reforecast database is now available that is approximately consistent with the operational 0000 UTC cycle of the 2012 NOAA Global Ensemble Forecast System (GEFS). The reforecast dataset consists of an 11-member ensemble run once each day from 0000 UTC initial conditions. Reforecasts are run to +16 days. As with the operational 2012 GEFS, the reforecast is run at T254L42 resolution (approximately 1/2° grid spacing, 42 levels) for week +1 forecasts and T190L42 (approximately 3/4° grid spacing) for the week +2 forecasts. Reforecasts were initialized with Climate Forecast System Reanalysis initial conditions, and perturbations were generated using the ensemble transform with rescaling technique. Reforecast data are available from 1985 to present.

Reforecast datasets were previously demonstrated to be very valuable for detecting and correcting systematic errors in forecasts, especially forecasts of relatively rare events and longer-lead forecasts. What is novel about this reforecast dataset relative to the first-generation NOAA reforecast is that (i) a modern, currently operational version of the forecast model is used (the previous reforecast used a model version from 1998); (ii) a much larger set of output data has been saved, including variables relevant for precipitation, hydrologic, wind energy, solar energy, severe weather, and tropical cyclone forecasting; and (iii) the archived data are at much higher resolution.

The article describes more about the reforecast configuration and provides a few examples of how this second-generation reforecast data may be used for research and a variety of weather forecast applications.

Full access